The equation showed that mass and energy were related and that one could, in theory, be transformed into the other. But because the speed of light squared is such a huge number, it meant that even a small amount of mass could potentially be converted into a huge amount of energy. Ever since the discovery of radioactivity in the late 19th century, scientists had realised that the atomic nucleus could contain a large amount of energy. Einstein's revolutionary equation showed them, for the first time, just how much there was.
However, at the time Einstein doubted whether that energy could ever be released. … (from bbc.co.uk)
Books and Films - Albert Einstein
E=MC^2: Einstein's Most Famous Equation